Warning: for JNC books these are the "standard" editions not the "premium" versions with the content listed here: https://forums.j-novel.club/topic/305/premium-e-book-bonus-contents-list/
If you want the bonus stories you need to become a JNC member and buy the DRM-free epubs directly from JNC.
It's very easy to copy the books to a kindle with the free Calibre ebook app. Attach kindle to PC with USB cable, add book to Calibre library, click on "send to device" button, wait a minute, done.
For phones you can just install an app that reads epub files.